Spark Plug Recommendation
Thread Starter
Racer
Joined: Jul 2007
Posts: 444
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From: Roselle IL
Spark Plug Recommendation
I have a '75 with a '69 BB that has an HEI Distributor. The book recommends an AC 43XL plugs for this engine with a .035 gap. I noticed at the start of HEI years ('75), the plug recommendations jumped from a 44 to a 45 and the gap jumped to a .040.
I assume the temp range changed due to emissions as the engines were going to 195 degrees. However, I assumed the gap change was due to the increased voltage from HEI.
Any suggestions on a plug and a gap for this engine. It is an L-88 at 12.5:1 if that makes a difference.
Thanks in advance for your assistance.
I assume the temp range changed due to emissions as the engines were going to 195 degrees. However, I assumed the gap change was due to the increased voltage from HEI.
Any suggestions on a plug and a gap for this engine. It is an L-88 at 12.5:1 if that makes a difference.
Thanks in advance for your assistance.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Compression ratio is a huge factor in determining spark plug heat range. Higher compression ratios call for cooler plugs, and the lower the AC plug number the cooler the plug is. For a 12.5:1 the 43's wouldn't tend to foul as much, but I'd try 42's and keep an eye out for fouling, moving back to 43's if it occurs. This is going to depend on how much of the time the engine is being run hard vs. loafing around.
As for the gaps, the higher the spark voltage the wider the gap it will span, but the higher the compression, the less so you can get away with. You're looking for the widest gap you can reliably fire. Here, that 12.5:1 definitely makes a difference. (Hope you're not attempting to run it on pump gas.) At a minimum, I'd upgrade that HEI with at least MSD's ultimate HEI kit (better, replace it with MSD's e-curve dizzy), and try to open that gap to at least .040". That should reliably supply an ample spark for that beast. If that works, go for .045". If that's too much, you can either close it back down or add an MSD 6AL.
In either case, heat range or gap, you trying to find the right balance.
Hope that helps get you going.
As for the gaps, the higher the spark voltage the wider the gap it will span, but the higher the compression, the less so you can get away with. You're looking for the widest gap you can reliably fire. Here, that 12.5:1 definitely makes a difference. (Hope you're not attempting to run it on pump gas.) At a minimum, I'd upgrade that HEI with at least MSD's ultimate HEI kit (better, replace it with MSD's e-curve dizzy), and try to open that gap to at least .040". That should reliably supply an ample spark for that beast. If that works, go for .045". If that's too much, you can either close it back down or add an MSD 6AL.
In either case, heat range or gap, you trying to find the right balance.
Hope that helps get you going.
Racer
Joined: Jun 2006
Posts: 316
Likes: 1
From: Southern California
I'm not the expert, but I'm running a 75 HEI distributor in my 73 L-48 with cam, intake and header upgrades.
From what I've read you're right about the HEI higher voltage allowing the gap to be wider. But the Rapidfire #10 plug still show a .035 gap for the 75 Vette which the Platinum and Standard plugs call for the .060 gap.
At .035 my performance has been fine, but I reserve judgement until I try somethem at a wider gap. Hopefully someone will chime in with some proven experience and state the pros and cons on the different plugs in a modified engine and ignition set up.
From what I've read you're right about the HEI higher voltage allowing the gap to be wider. But the Rapidfire #10 plug still show a .035 gap for the 75 Vette which the Platinum and Standard plugs call for the .060 gap.
At .035 my performance has been fine, but I reserve judgement until I try somethem at a wider gap. Hopefully someone will chime in with some proven experience and state the pros and cons on the different plugs in a modified engine and ignition set up.
Thread Starter
Racer
Joined: Jul 2007
Posts: 444
Likes: 0
From: Roselle IL
Compression ratio is a huge factor in determining spark plug heat range. Higher compression ratios call for cooler plugs, and the lower the AC plug number the cooler the plug is. For a 12.5:1 the 43's wouldn't tend to foul as much, but I'd try 42's and keep an eye out for fouling, moving back to 43's if it occurs. This is going to depend on how much of the time the engine is being run hard vs. loafing around.
As for the gaps, the higher the spark voltage the wider the gap it will span, but the higher the compression, the less so you can get away with. You're looking for the widest gap you can reliably fire. Here, that 12.5:1 definitely makes a difference. (Hope you're not attempting to run it on pump gas.) At a minimum, I'd upgrade that HEI with at least MSD's ultimate HEI kit (better, replace it with MSD's e-curve dizzy), and try to open that gap to at least .040". That should reliably supply an ample spark for that beast. If that works, go for .045". If that's too much, you can either close it back down or add an MSD 6AL.
In either case, heat range or gap, you trying to find the right balance.
Hope that helps get you going.
As for the gaps, the higher the spark voltage the wider the gap it will span, but the higher the compression, the less so you can get away with. You're looking for the widest gap you can reliably fire. Here, that 12.5:1 definitely makes a difference. (Hope you're not attempting to run it on pump gas.) At a minimum, I'd upgrade that HEI with at least MSD's ultimate HEI kit (better, replace it with MSD's e-curve dizzy), and try to open that gap to at least .040". That should reliably supply an ample spark for that beast. If that works, go for .045". If that's too much, you can either close it back down or add an MSD 6AL.
In either case, heat range or gap, you trying to find the right balance.
Hope that helps get you going.
Thanks for the tips here. The engine has an Accel coil and the 43XLS's don't look fouled, so maybe I'll try the 42's. It runs on 100LL or 100 pump gas.
The previous owner had problems with fouling and someone told him to replace the 4.10 rear end with a 3.70, so he did. Maybe it was the plugs. This should be an interesting trial and error session for me.
Thanks again for the advice.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Since your 43's are looking good, you may only want to try the 42's at the track when you're running hard and keep the 43's for any cruising.
On your gap, when you no longer see any perf gains from opening them up, close them back down 0.005" and you'll be there.
One more thing you can do to optimize your plugs is to dress the electrodes as on the left below. Indexing them may also help, so I do that too, knowing I've left nothing on the table
On your gap, when you no longer see any perf gains from opening them up, close them back down 0.005" and you'll be there.
One more thing you can do to optimize your plugs is to dress the electrodes as on the left below. Indexing them may also help, so I do that too, knowing I've left nothing on the table
Last edited by TheSkunkWorks; Aug 9, 2007 at 05:25 PM.
Instructor
Joined: Jun 2006
Posts: 104
Likes: 0
From: Missouri City Texas
http://www.idavette.net/tech/acplugs.htm
Try this website...I am using the RF#2, and they wrok great, but I do not have your hi compression ratio.
Try this website...I am using the RF#2, and they wrok great, but I do not have your hi compression ratio.
